Integrated Colour LED Micro-Display

ABSTRACT

There is herein described a low power consumption high brightness display. More particularly, there is described an integrated LED micro-display and a method of manufacturing the integrated LED micro-display.

FIELD OF THE INVENTION

The present invention related to a low power consumption high brightnessdisplay. More particularly, the present invention related to anintegrated colour LED micro-display and a method of manufacturing theintegrated colour LED micro-display.

BACKGROUND OF THE INVENTION

Although there are many colour micro-displays many of the prior artcolour micro-displays have a number of disadvantages.

There is extensive literature reporting the development ofmicro-displays using technologies such as OLED, Liquid Crystal and MEMS.The latter two are based on pattern generators located externally to alight source that is permanently on full brightness, and consequentlyrequire extra components to form the micro-display. A further basicdrawback then related to the power loss as all pixels must be addressedwith light even if they are not used to display the image. The contrastratio of such displays is also compromised.

OLED technology is an emissive technology and in simple terms is basedon an anode and cathode surrounding a fluorescent emitting layer. Thesetechniques often use white light with colour filters for small pixelformation. Consequently, approximately 60 to 70% of the spectral rangeof the white pixels are lost/not needed to achieve the colour gamut in aRGB display. In addition, white OLEDs are in themselves less efficientthan monochrome OLEDs, such that in the end only 10 to 20% of theemitted light can actually be used. This does not take into account forthe efficiencies of the overall OLED structure or how the light isextracted.

Moreover, the OLED structure is also more complex and involves electrontransport layer, hole blocking layer and electron blocking layer allcarefully controlled in thickness and refractive index. This isimportant for improved display performance as the electrically dopedelectron and hole transport layers enable enhanced charge injection andlow operating voltages. The charge blocking layers help to confinecharge carriers within the emission layer. Furthermore, other issuesrelate to the poor efficiencies and limited lifetimes in the blue OLEDwavelength region and coupled with the low brightness levels mean thatthe display has fundamental limitations in performance.

Techniques do exist to provide surface mount bonding in individual LEDs.Typically, pick and place techniques can only be used for large LEDs.Thus limiting the pixels per inch for a display. It also means thatthere is the need for two electrical contacts per pixel. For the formerpoint, techniques have been developed to pick and place micro-LEDs.However, to provide electrical contacts presents challenges for smallpixel with post-processing required.

Disadvantages of such systems can be summarised by the following:

-   -   Manufacturing—time per flip-chip bond, simultaneous n and p        connection for each pixel and ability to place pixels with <10        μm dimensions;    -   Post processing of pick and place micro-LEDs using semiconductor        processing techniques. The provision of conformed contact layers        across the LED arrays to form a secondary global contact. Or a        combination of planarization techniques to provide a planarised        structure on which a patterned contact layer is formed. The need        to provide transparent contact layers for light escape or        subsequent patterning of contact layers to enable this. The need        to provide electrical connection to the control backplane.    -   Performance—in particular selection of green LED devices with        small chromatic variation over drive current and temperature.        Requirement to have each green LED emission wavelength in a        tight distribution due to the eye's sensitivity to small        variations in wavelengths near the peak of its visual response        (i.e. green).

It is an object of at least one aspect of the present invention toobviate or mitigate at least one or more of the aforementioned problems.

It is a further object of at least one aspect of the present inventionto provide a low power consumption high brightness display and a methodof manufacturing said display.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided amethod of fabricating an integrated LED micro-display comprising:

-   -   providing a colour converter capable of changing the wavelength        of light;    -   providing an array of micro-LEDs connected to the colour        converter and which is capable of forming electrical connections        and pumping light into the colour converter; and    -   providing a backplane control in the form of an electronic drive        layer;    -   wherein the micro-LED array generates pumped light at a shorter        wavelength than emitted light from the colour converter thereby        producing light at a longer wavelength.

The process of manufacturing involves a number of different stages whichare set out below. It should also be apparent that in variousembodiments description is made with reference to figures. However,certain embodiments may be practiced without one or more of thesespecific details, or in combination with other known methods andconfigurations.

First of all, there may be provided a GaN layer comprising p and n dopedGaN regions and layers optimized for efficient light generation on topof which there is an ohmic current spreading layer and a layer ofsilicon dioxide. Located below the GaN layer there may be a substratelayer which is significantly thicker than the other layers. The ohmiccurrent spreading layer may have a thickness of 20 nm. The layer ofsilicon dioxide may have a thickness of about 200 nm. The substratelayer may have a thickness of about 200 μm. The substrate layer may beany suitable material such as sapphire, silicon, GaN or silicon carbide.Example materials for the ohmic current spreading layer may be Ni/Au orNi/Pt or Au/Pt or Pt/Ni/Au Ni/Ag or Pd or ITO or Ni/ITO.

The process may start with selective deactivation of p-GaN to form apixel or an array of pixels. This can be described as follows:

-   -   a first spreading layer is deposited on top of a GaN p layer        e.g. Ni/Au    -   a patterned mask feature (e.g. photoresist) is then deposited on        the spreading layer    -   said structure is then exposed to a plasma such as Ar to remove        the spreading followed by Cl₂ to etch down ˜1 um to the n-GaN        (this process may be at a later stage in the process)    -   a patterned mask feature (e.g., photoresist) is then deposited        for pixel definition    -   then exposing the layered structure to an etch (plasma or dry)        to remove the first spreading layer from the areas not protected        by the mask e.g. Ar    -   said structure is then exposed to a plasma such as CHF₃    -   the removal of a patterned feature may then be performed    -   then annealing of the structure to form highly resistive layers        in the areas exposed to plasma, whilst retaining conductive e.g.        ohmic contract at the layers protected by the mask to form a        pixel or an array of pixels

Alternatively, the process may start with a pixel or an array of pixelformation using physical etching of the p-GaN. This can be described asfollows:

-   -   a first spreading layer is deposited on top of a GaN p layer        e.g. Ni/Au    -   a patterned mask feature (e.g. photoresist) is then deposited on        the spreading layer    -   said structure is then exposed to a plasma such as Cl2 to etch        down ˜1 um to the n-GaN to leave pixels    -   the removal of the patterned feature may then be performed    -   then annealing of the structure to form conductive e.g. ohmic        contact at the a pixel or an array of pixels

The layer of silicon dioxide on the top of the pixels may then beremoved and then re-deposited in the form of a complete layer of silicondioxide. The complete layer of silicon dioxide may have a thickness ofabout 200 nm and may be deposited using any suitable technique such asPECVD.

The area of the complete layer of silicon dioxide above the pixels maythen be etched away to form windows to the ohmic current spreading layerand a window where the common contact is to be formed. In thisembodiment this is the common n contact region. For the deactivationprocess the silicon dioxide must be formed on the sidewall to providepassivation. For the physical etched pixel or pixel array thepassivation of the sidewall is required where the n-contact region isetched locally. In addition, an n contact metal layer may be depositedin the area of the etch. The n contact metal may be Ti/Au and may have athickness of about 50/250 nm. The n contact metal layer may form aglobal contact and has an electrical function and/or functions as aguide for controlling a further etch step (an etch stop) in themanufacturing process.

Bond pads may then be deposited to form n and p contacts. The bond padsmay have a height of about 2 μm and a cross-section of about 8 μm×8 μm.It is the intention to have the p and n bond pads of the same height.Furthermore, the bond pads can be formed on a masking layer such thatthe metal is deposited everywhere. This then this provide a means forchemical polishing of the device to provide a flat top layer with themetal and dielectric planar. Such a top structure then provide the meansfor uniform bonding (p and n bond pad stacks) at the same height.Consequently, a range of bonding techniques can be utilized for GaN tobackplane control (CMOS, TFT or NMOS layer) including but not limited toflip-chip bump bonding and direct bonding based on Van der Waal'sforces. The latter is of particular interest as it permits bonding atlow temperatures and with very little bond force. As the LED array sizeincreases, the total bond force required becomes an importantcharacteristic and can result in a physical limitation on array size.Also because of the low temperature bond. TFT backplane structures canbe used which are similar in nature to Active Matrix OLED (AMOLED)control backplanes. This reduces the cost, complexity and provides aroute to displays with larger physical dimensions.

A temporary wafer made from Silicon may be bonded to the GaN wafer. Thetemporary wafer may have a thickness of about 500 μm and issignificantly thicker than the GaN layer.

The substrate layer may then be removed off with any suitable techniquesuch as laser lift-off. In the event that the substrate layer issilicon, chemical mechanical polishing and etching or a combination ofthe techniques is possible. This has the added capability of formingmicron features in the silicon substrate which can be used in theoverall design.

The buffer or n-GaN layer may then be etched to form optical featureswhich in the preferred embodiments may be in the form of micro-lenses.The optical features may be convex in shape to maximize emission andminimise optical crosstalk between pixels. The optical features may havea width of about 8 μm.

In an alternative, the etching process may form roughened areas inproximity to the pixel. The roughened areas of 1 μm in depth may be usedto improve light extraction.

In a further alternative there may be a matrix etching process where GaNpillars may be formed. The GaN pillars may have a height of about 2 μm.The GaN pillars are also preferably truncated in shape to maximizeoptical isolation between LED pixel output to reduce optical crosstalk.

The n-side etch process may be accurately controlled by etching to the ncontact metal layer on the other side of the wafer to provide a suitableetch stop. By using real time plasma monitoring technique such as endpoint detection the etching depth can be accurately controlled inrespect to the position of the GaN quantum well.

In a further alternative the n contact layer may be used as an etch stopto control the lens thickness. The end point detection may be usedduring the lens etch to simultaneously open through to the n contactmetal layer and provide control of the lens thickness.

There may then be a deposition step where there may be a deposition ofconductive metal grid and/or opaque layer. The conductive metal gridand/or opaque layer may reduce n-contact resistance hence and has ashielding effect. The metal grid and/or opaque layer may have athickness of about 200 nm and may be deposited using any suitabletechnique.

In an alternative an ITO conductive layer may be deposited instead ofmetal to create a transparent conductive layer. The selection of theappropriate ITO thickness may result in an anti-reflection coating toincrease light transmission.

In a further alternative if GaN pillars are formed then they may becoated with a layer of a metal to cover the sidewalls to minimizecrosstalk. The layer of n metal may have a thickness of about 200 nm.

A layer of SiO₂ or any other suitable type of single or multi-layercoating may also be deposited over the surface of the GaN lens and the nmetal layer. This provides both protection and an anti-reflectionfunction of the GaN surface as it reduces Fresnel reflection. Thecoating may have a thickness of about 80 nm.

In a further alternative only the curved area of the optical features,or roughened areas may be covered with a dielectric coating. Thedielectric coating may be made from silicon dioxide and may have athickness of about 80 nm.

In a further alternative coatings may be deposited. There may thereforebe multi-layer dielectric coatings. There may simply be one coating oradditional coatings or layers which have the function of providing ashort pass wavelength filter function. The coatings may be patterned tocover the optical features only or it may cover the full surface. Theshort pass filter may be designed to allow blue light to exit the GaNbut reflect longer wavelengths (i.e. the red or green light generated bythe colour conversion layer).

The process also requires use of a colour converter which in someembodiments can have high reflectivity layers adjacent to sub-pixels toimprove colour conversion efficiency and display contrast. The colourconverter comprises a colour conversion layer, a substrate, atransparent layer and a mask. The colour conversion layer may be madefrom phosphor, quantum dot, organic substance or a combination thereof.The colour conversion layer may have a thickness of about 1-20 μm orpreferably about 1-10 μm. The substrate may be made from glass,sapphire, silicon or any other suitable material. If blue light is usedto optically pump the colour converter then it is not necessary toconvert the incident light for the blue sub-pixel of the display.Consequently there may be no material in the blue cell or a transparentlayer which may be made from silicon and allows blue light to exit oralternatively may provide a diffusing or scattering function to providea similar beam profile as the red and green phosphors. The mask may beopaque/black matrix resin (commonly used in LCD displays) or reflectiveand may be made from Au, Al or Ag.

In an alternative colour converter there may be an etched siliconsubstrate. The colour converter may compromise a colour conversionlayer, a transparent layer and an etched silicon area.

In an alternative colour converter there may be a short pass filterwhich allows blue light to enter, but reflects longer converted lightsuch as green and red.

In a further alternative there may be a colour converter which can bedescribed as operating like a waveguide. As before there is a colourconversion layer, transparent layer and a mask. In addition there areopaque/reflective features, a modified refractive index transparentlayer which provides an optical waveguiding function and an un-modifiedrefractive index layer.

In a further alternative there is a colour converter which can bedescribed as a long pass filter. The colour converter may have a longpass filter located below the colour conversion layer. The long passfilter may allow converted light to exit, but recycles blur unconvertedlight.

In the next step in the process the colour converter is brought up toand aligned with the LED layer.

The colour converter may then be attached to the rest of the device. Ina preferred embodiment the colour converter may be pumped with bluelight and there is a red/green colour conversion layer which is thephosphor layer. Alternatively, the layer may be a quantum dot ortransparent/diffusing layer (blue) or a mixture thereof. Extendingvertically down from the glass substrate there may be an opaque/blackmask or reflective masks. The reflective mask may be preferred as thishas the ability to recirculate the light, minimises crosstalk andenhances display contrast. The reflective mask therefore has the abilityto transmit blue light and reflect red and green light when a filter isplaced before the transmittive layer. In an alternative, if the filteris placed after the transmittive layer then the blue light will berecirculate and the red and green transmitted.

In the next stage in the process the temporary layer may be removed.

The electronic drive layer may then be brought up to and aligned andattached to the metal bond pads on the LED. The bonding layer stack mayinclude a low temperature solder material such as tin or indium, oralloys thereof. The planarity of the GaN p-layer also provides thecapability to use low temperature direct bonding, including van derWaals forces, hydrogen bonds and strong covalent bonds. The electronicdrive layer may be a CMOS, TFT or NMOS NMOS layer. In particular, theability to use direct bonding techniques permits the ability to adaptthin film transistor techniques employed in matrix organic lightemitting diode (AMOLED) micro-displays.

The integrated colour LED micro-display may have a pixel layoutimplementation. In one embodiment, three sub-pixels may be arranged in a20×20 micron cell with the bond pads being positioned away from thepixels.

In an alternative embodiment, the pixel layout implementation maycomprise four sub-pixels included in each pixel with the bond padspositioned over the pixels.

The integrated colour LED micro-display may comprise a red colourconversion cell, a green colour conversion layer and a blue pixelwithout colour conversion cell (which may include atransparent/diffusing layer).

In an alternative integrated colour LED micro-display there may be anopaque/reflecting matrix.

In an alternative integrated colour LED micro-display there may be amatrix of red colour conversion cells, green colour conversion cells andblue pixels without colour conversion cells.

In a yet further alternative integrated colour LED micro-display theremay be a matrix of red colour conversion cells, green colour conversioncells, blue pixels without colour conversion cells and anopaque/reflective matrix.

According to a second aspect of the present invention there is anintegrated colour LED micro-display comprising:

-   -   a colour converter capable of changing the wavelength of light;    -   an LED connected to the colour converter and which is capable of        forming electrical connections and pumping light into the colour        converter; and    -   a back plate control in the form of an electronic drive layer;    -   wherein the LED pumps light at a shorter wavelength than emitted        light from the colour converter thereby producing light at a        longer wavelength.

Generally speaking, the present invention therefore resides in theprovision of an integrated colour LED micro-display which provides a lowpower consumption high brightness display.

The integrated colour LED micro-display may be formed as defined in thefirst aspect.

The colour converter may comprise a colour conversion layer, asubstrate, a transparent layer and a mask. The colour conversion layermay be made from phosphor, quantum dot, organic substance or acombination thereof. The colour conversion layer may have a thickness ofabout 5-20 μm or preferably about 10-12 μm. The substrate may be madefrom glass, sapphire, silicon or any other suitable material. Thetransparent layer may allow blue light to exit or alternatively mayprovide a diffusing or scattering function. The mask may be opaque/blackor reflective.

The LED may comprise optical features which are in the form of lenses.The optical features may be convex in shape and maximize emission andminimise spectral crosstalk.

The LED may also comprise a musk which may be opaque/black or reflectiveand may be made from metal such as Al or resin/polymer.

The LED may also comprise bond pads which are deposited to form n and pcontacts The bond pads may have a height of about 2 μm and across-section of about 8 μm×8 μm.

The backplane control may comprise an electronic drive layer which hasbond pads. The electronic drive layer may be a CMOS, TFT or NMOS NMOSlayer.

The bond pads on the backplane control are attached to bond pads on theLED.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 represents a GaN layer on top of which there is an ohmic currentspreading layer and a layer of silicon dioxide and below there is asapphire substrate layer according to an embodiment of the presentinvention;

FIG. 2 represents an alternative processing method where a GaN layer hasetched pixels comprising a layer of silicon dioxide and ohmic currentspreading layer according to an embodiment of the present invention;

FIG. 3 represents an etching process performed on the device shown inFIG. 1 according to an embodiment of the present invention;

FIG. 4 represents an alternative device to that shown in FIG. 2 which isetched in the GaN layer around pixels to form an etched area accordingto an embodiment of the present invention;

FIG. 5 represents a further etching process where the ohmic currentspreading layer and the layer of silicon dioxide are etched away to formpixels according to an embodiment of the present invention;

FIG. 6 represents a plasma treatment which is used to create conductiveand insulating regions according to an embodiment of the presentinvention;

FIG. 7 represents the layer of silicon dioxide on the top of the pixelsbeing removed and then re-deposited in the form of a complete layer ofsilicon dioxide according to an embodiment of the present invention;

FIG. 8 represents the area of the complete layer of silicon dioxideabove the pixels being etched away to form contact windows according toan embodiment of the present invention;

FIG. 9 represents bond pads deposited to form n and p contacts accordingto an embodiment of the present invention;

FIG. 10 represents the substrate layer being removed according to anembodiment of the present invention;

FIG. 11 represents the GaN layer being etched to form optical featuresaccording to an embodiment of the present invention;

FIG. 12 represents an alternative etching method to form roughened areasaccording to an embodiment of the present invention;

FIG. 13 represents a further alternative to the etching method where thematrix etching process forms GaN pillars according to an embodiment ofthe present invention;

FIG. 14 represents a further alternative to the etching method where theGaN layer is etched through to form an etch to an n contact metal layeraccording to an embodiment of the present invention;

FIG. 15 represents a further alternative where the n contact layer isused as an etch stop to control the lens thickness according to anembodiment of the present invention;

FIG. 16 represents a deposition step where there is a deposition ofconductive metal grid and/or opaque layer according to an embodiment ofthe present invention;

FIG. 17 represents an alternative deposition step where an ITOconductive layer is deposited instead of metal to create a transparentconductive layer according to an embodiment of the present invention;

FIG. 18 represents a further alternative where GaN pillars are coatedwith a layer of n metal according to an embodiment of the presentinvention;

FIG. 19 is an upper perspective view of the device shown in FIG. 16according to an embodiment of the present invention.

FIG. 20 represents the process where a layer of SiO₂ or any othersuitable type of single or multi-layer coating is deposited over thesurface of the GaN lens and the n metal layer according to an embodimentof the present invention;

FIG. 21 represents a further alternative where only the curved area ofthe optical features are covered with a dielectric coating according toan embodiment of the present invention;

FIG. 22 represents is a further alternative where coatings are depositedto have the function of providing a short pass wavelength filterfunction according to an embodiment of the present invention;

FIG. 23 represents a colour converter according to an embodiment of thepresent invention;

FIG. 24 represents an alternative colour converter where there is anetched silicon substrate according to an embodiment of the presentinvention;

FIG. 25 represents a further alternative colour converter which is ashort pass filter according to an embodiment of the present invention;

FIG. 26 represents a further alternative colour converter which can bedescribed as operating like a waveguide according to an embodiment ofthe present invention;

FIG. 27 represents a further alternative colour converter which can bedescribed as a long pass filter according to an embodiment of thepresent invention;

FIG. 28 represents the colour converter being brought up to and alignedwith an LED layer according to an embodiment of the present invention;

FIG. 29, represents a further view of the colour conversion layer beingaligned above the rest of the LED layer;

FIG. 30 represents a colour converter attached to the rest of the deviceaccording to an embodiment of the present invention;

FIG. 31 represents a temporary layer being removed according to anembodiment of the present invention;

FIG. 32 represents an electric drive layer being brought up to andaligned with metal bond pads according to an embodiment of the presentinvention;

FIG. 33 represents metal bond pads being attached to the bond padsaccording to an embodiment of the present invention;

FIGS. 34 and 35 represent a plan view of pixel layout implementationaccording to an embodiment of the present invention;

FIGS. 36 and 37 represents a plan view of a further pixel layoutimplementation according to an embodiment of the present invention;

FIG. 38 represents a red colour conversion cell, a green colourconversion layer and a blue pixel without colour conversion cellaccording to an embodiment of the present invention;

FIG. 39 represents a colour conversion cell where there is anopaque/reflecting matrix according to an embodiment of the presentinvention;

FIG. 40 represents a matrix of red colour conversion cells, green colourconversion cells and blue pixels without colour conversion cellsaccording to an embodiment of the present invention; and

FIG. 41 represents a matrix of red colour conversion cells, green colourconversion cells, blue pixels without colour conversion cells and anopaque/reflecting matrix according to an embodiment of the presentinvention;

BRIEF DESCRIPTION

Generally speaking, the present invention resides in the provision of alow power consumption high brightness display.

FIGS. 1 to 41 show the process of making a micro-display according tothe present invention. This is discussed below.

FIG. 1 shows a light emitting GaN layer 3 on top of which there is anohmic current spreading layer 2 and a layer of silicon dioxide 1.Located below the GaN layer 3 there is a substrate layer 4 which issignificantly thicker than the other layers. The ohmic current spreadinglayer 2 has a thickness of about 20 nm. The layer of silicon dioxide 1has a thickness of about 200 nm. The substrate layer 4 has a thicknessof about 200 nm. The substrate layer 4 can be any suitable material suchas sapphire, silicon, GaN and silicon carbide. Example materials for theohmic current spreading layer 2 are Ni/Au or Ni/Pt or Au/Pt or Pt/Ni/AuNi/Ag or Pd or ITO or Ni/ITO.

FIG. 2 represents an alternative process method where the GaN layer 3has etched pixels comprising a layer of silicon dioxide 1 and ohmiccurrent spreading layer 2. This is a standard technique to form the LEDpixel. In FIG. 2 the spreading layer 2 is defined, typically to be thesame size as the final pixel dimension.

In FIG. 3 an etching process is performed on the device shown in FIG. 1which is a GaN modification process. The etching produces an etched area5 where the GaN layer 3 is etched. The etching process is performedusing any suitable etching process but is preferably performed by plasmaetching using CF₃ but other techniques such as wet etching may be used.Etching down to the n material may be performed using plasma etchingusing Cl.

In FIG. 4 the alternative device shown in FIG. 2 is etched in the GaNlayer 3 around the pixels to form an etched area 5.

As shown in FIG. 5 a mask is then applied so as to enable pixelformation. There is a further etching process where the ohmic currentspreading layer 2 and the layer of silicon dioxide 1 are etched away toform pixels 6. The etching process is performed using any suitableplasma etching process such as CHF₃. The pixels 6 can be formed in amatrix pattern. This leaves pixels with dimensions of between 0.5 μm and100 μm. A typical dimension is about 3 μm.

In the process step shown in FIG. 6, there is a plasma treatment 7 whichis used to create conductive and insulating regions. The unprotected GaNis exposed to a GaN modification process e.g. a plasma such as CHF₃. Theremoval of the patterned feature may then be performed followed byannealing of the structure to form highly resistive layers in the areasexposed to plasma, whilst retaining conductive e.g. ohmic contact at thelayers protected by the mask to form a pixel or an array of pixels.

In FIG. 7 a mask layer is formed. The layer of silicon dioxide 1 on thetop of the pixels 6 is removed and then re-deposited in the form of acomplete layer 8 of a photoresist only or a dielectric e.g. SiO₂ withphotoresist on top to pattern the SiO₂ layer.

In FIG. 8, the mask is then opened above the n-contact area and thespreading material. A conductive contact is then deposited e.g. Ti:Au orother combinations. It should be apparent that the etch sidewall at then-contact area has an electrical insulating layer on the sidewalls toprevent shorting across the p-n junction. The area of the complete layer8 of silicon dioxide above the pixels 6 is etched away to form contactwindows 10 to the ohmic current spreading layer 2 below to createcontact. In addition, an n contact metal layer 9 is deposited in thearea of the etch 5. The n contact metal layer 9 forms a global contactand has an electrical function and/or functions as a guide forcontrolling a further etch step in the manufacturing process.

In FIG. 9, bond pads 11 are deposited to form n and p contacts. This mayuse the same masking layer or alternatively a new mask. The p and n bondpad have the advantage of having the same height and thus improve thebonding probability of success for an array of LEDs. The n-pad etchdepth can be used as both an accurate etch stop for the process later oni.e. when etching from the GaN side, exposed after laser lift off, andas a means to provide a connection for a distributed electrical contacton the backside of the wafer. Furthermore, the mask layer for formingthe bond pad metal can provide a layer without topology and as such aprocess such as CMP—chemical mechanical polishing (damscene) can be usedto planarise the metal to the top of the mask layer. This polishingprocess then enables the ability to have a flat bonding surface.Consequently, it will be possible to use a range of bonding processesincluding but not limited to flip-chip bump bonding (thermo-sonic orthermo-compression), or direct bonding or any other technique to form amechanical and electrical bond to a backplane, e.g. CMOS, TFT or NMOSlayer.

As shown in FIG. 10, the substrate layer 4 is removed off with anysuitable technique such as laser lift-off. The substrate layer 4 canthen undergo silicon, chemical mechanical polishing and etching for thesilicone. The GaN on substrate (sapphire, etc.) is then bonded to atemporary wafer with the GaN surface in contact to this layer. Industrystandard techniques may be used to detach the GaN layer from thesubstrate. In this particular example laser lift off is used.Alternatively, CMP can be used to remove silicon of GaN. This can thenbe combined with etching of the substrate so that features can be etchedinto the substrate. As is shown later (FIG. 13) pillars can be left fromthe Si to provide isolation between sub-pixels. The temporary wafer 13has a thickness which is significantly thicker than the GaN layer 3.

In FIG. 11, now the GaN layer is exposed on the backside a range offeatures can be formed in this structure. This can be to provide themeans of extracting more light and/or providing features to reduceoptical cross-talk between sub-pixels. The GaN layer 3 is etched to formoptical features 14 which are in the form of lenses. The opticalfeatures 14 are convex in shape and maximize emission spectralcrosstalk. It is also possible to deposit a layer on these features suchas SiO₂ to act as an anti-reflection coating. In this particularfeature, lenses are formed (the height can be accurately determined byusing the etch stop) to increase light extraction and to reduce opticalcrosstalk between sub-pixels.

As shown in FIG. 12, in an alternative the etching may be used to formroughened areas 15. The roughened areas 15 may be used to improve lightextraction. In this case a scattering surface is formed from etching.Like FIG. 11 a layer or layers can be deposited to reduce the Fresnelreflections.

FIG. 13 is a further alternative where in the matrix etching process,GaN pillars 16 are formed. As described in FIG. 10, pillars 16 cantherefore be formed between the light extraction features (in thisexample lenses). This can be formed in the GaN at the same time as thelight extraction features using standard techniques. Alternatively, thepillars 16 can be formed in the original substrate (description in FIG.10) or they can be formed in the colour conversion substrate (describedin FIG. 23). The pillars 16 will ideally have smooth surfaces and ahighly reflective structure, irrespective of the technique.

The process as shown in FIG. 14 then involves a further etching processwhere the GaN layer 3 is etched through to form an etched area 17 to then contact metal layer 9 over the etched area 5. In this particularinstance the light extraction feature (lens) and n opening can besimultaneously formed. The etching is performed by any suitable etchingtechnique.

FIG. 15 shown an alternative where the n contact layer 9 is used as anetch stop to control the lens thickness. The end point detection is usedduring the lens etch 18 to simultaneously open through to the n contactmetal layer 9 and provide control of the lens thickness. As described inFIG. 9, the performed embodiment for the FIG. 14 process is that then-contact can be used as an etch stop so as to accurately control theetch depth of the lens and thus its proximity to the QW layers.

FIG. 16 then shows that there is a deposition step where there is adeposition of conductive metal grid and/or opaque layer 19. This issimilar to as described in FIG. 9 and the described ability to form adistributed electrical contact on the exposed GaN face. In this instancethis forms an electrical bridge between the bond pad and the conductiveGaN face. In this instance an opaque conductor is used. Consequently,this is not deposited over the light extraction features. The conductivemetal grid and/or opaque layer 40 reduces bias and has a shieldingeffect.

FIG. 17 represents an alternative where an ITO conductive layer 20 isdeposited instead of metal to create a transparent conductive layer 20.The selection of the appropriate ITO thickness may result in ananti-reflection coating. Like FIG. 16 a distributed n-contact can beformed. However, in this instance a transparent ITO layer can bedeposited uniformly across the surface (can also be patterned so as notcovering the light extraction feature). This can also be designed tohave a thickness to reduce Fresnel reflections.

FIG. 18 is a further alternative where the GaN pillars 16 are coatedwith a layer 21 of n metal and may also cover the sidewalls to minimizecrosstalk. This is a combination of FIGS. 10 and 17 where the metal canbe patterned to provide distributed electrical contact and also providereflective pillars.

FIG. 19 is a schematic to illustrate the principle of forming lightextraction features, an etch stop, distributed n contact and all bondedto a temporary carrier wafer such as silicon. This summarizes thecompleted GaN chip which can then be integrated to a backplane controlsubstrate and a colour converting substrate.

In FIG. 20 a layer of SiO₂ or any other suitable type of single ormulti-layer coating 22 may be deposited over the surface of the GaN lens14 and the n metal layer 19. This provides both protection and ananti-reflection function.

In FIG. 21 there is a further alternative where only the curved area ofthe optical features 14 (or roughened areas 15) are covered with adielectric coating 23. Patterning is therefore only used to deposit overthe light extraction area.

In FIG. 22 there is a further alternative where coatings 24 a, 24 b maybe deposited. There may therefore be multi-layer dielectric coatings.There may simply be one coating or additional coatings or layers whichhave the function of providing a short pass wavelength filter function.The coatings 24 a, 24 b may be patterned to cover the optical features14 only or it may cover the full surface. The short pass filter may bedesigned to allow blue light to exit the GaN but reflect longerwavelengths (i.e. the red or green light generated by the colourconversion layer). Like FIGS. 20 and 21 one of the great advantages ofpresent technique is that many different layers can be deposited overthe light extraction area. In this particular arrangement a short passfilter is employed. This efficiently transmits light in the pumpwavelength region e.g. blue. It then reflects longer wavelength i.e.green and red so that the converted light which gets back to the GaNsurface has a higher probability of exiting in the direction of theend-user.

In FIG. 23 shows that now that the GaN layer has been prepared the nextstage is to form a colour convertor region. The preferred route is touse a separate substrate as this provides enhanced flexibility, althoughcolour convertors could be place on the GaN surface. In this case thecolour convertor may be a phosphor, quantum dot, organic substance or acombination. The colour convertor substrate can be a range of materialssuch as glass, sapphire, silicone, etc. In this instance each colourconvertor provides a sub-pixel and is optically pumped (blue—preferableor UV light). In the case a blue pump wavelength is used the bluesub-pixel has no colour convertor substance but may have a material tomimic the colour convertor so that is has similar emission propertiese.g. beam divergence. In the event that each LED pump is UV then thereis a colour convertor for red, green and blue. A matrix is also formedbetween the sub-pixels for improved contrast and to prevent lightleakage into the adjacent pixel. This may be a black matrix or areflective structure. Typically the matrix is formed on the colourconversion substrate. The colour conveyer comprises a colour conversionlayer 25, a substrate 26, a transparent layer 27 and a mask 28. Thesubstrate 26 may be made from glass, sapphire, silicon or any othersuitable material.

FIG. 24 represents the option for forming the matrix for the colourconverter region. In the event that the GaN on silicon substrate routeis used, it is possible to easily polish and etch the silicon substrate(for other substrate such as sapphire or SiC this is a much moredifficult process). Consequently, it is possible to etch a matrix intothe silicon using wet or dry etching capabilities. Using either etchingtechniques can form matrices with high aspect ratio i.e. height to widthof structure. The silicon will absorb light in the visible wavelengthregion, thus enhancing contrast. It is also possible to metallise thesilicon matrix to provide reflection and enhance lightconversion/output. The colour converter comprises a colour conversionlayer 25, a transparent layer 27 and an etched silicon area 29.

FIG. 25 is a further alternative colour converter. To enhanceperformance it is possible to include a filter on the colour converterlayer. The short pass filter transmits light in blue and reflects atlonger wavelengths i.e. green and red. For simplicity a multi-layercoating can be formed on all cells. Consequently, the light converted,which is isotropic in nature, is reflected and will exit in the intendeddirection. In the case of the blue emission pixel it may or may not beappropriate to have the filter. If pumping with UV light then the filterwill be placed over all cells and have the properties of transmittingthe UV pump but reflecting the longer wavelengths.

FIG. 26 is a further alternative colour converter which can be describedas operating like a waveguide. As before there is a colour conversionlayer 25, transparent layer 27 and a mask 28. In addition there areopaque/reflective features 33, a modified refractive index transparentlayer 34 which provides an optical waveguiding function and anun-modified refractive index layer 35. For the colour convertertransparent substrate (on which the colour convertors and matrix isformed), it is possible to modify the transparent layer 27. This canenable waveguides normal to the substrate plane to be formed.Consequently, the higher index layers will enhance optical waveguidingand enable the light to exit with a lower divergence. The index of thesubstrate can be modified as an example by laser induced effects to formthe waveguide or can be formed by the etching and filling using higherindex material. It is possible to use a transparent layer 34 of standardthickness of 0.5 to 2 mm. Furthermore, it is possible to use substratelayers 34 with thicknesses down to 20 μm.

FIG. 27 is a further alternative colour converter which can be describedas a long pass filter. A further option/embodiment for the colourconvertor transparent substrate is to also deposit a long pass filter,prior to the colour convertor being deposited on the substrate. Thisworks in a similar fashion as FIG. 24 except in this arrangement theunconverted blue light is reflected back to the colour conversionregion. The colour convertor is patterned so as not to cover the bluesub-pixel. The colour convertor has a long pass filter 31 located belowthe colour conversion layer 25. The long pass filter 31 allows convertedlight to exit, but recycles blue unconverted light. FIG. 27 also showsthat a long pass filter 32 would not be deposited over blue pixels toallow transmission of blue light.

In the next step shown in FIG. 28, a colour converter is brought up toand aligned with the LED layer. In this diagram the colour conversionlayer is therefore aligned to the LED substrate. This is in preparationfor bonding of the two structures and is completed in such a manner thatthe GaN sub-pixel LEDs are aligned to the respective colour conversionregion.

In FIG. 29, there is a further view of the colour conversion layer beingaligned above the rest of the LED layer. This is a schematic of FIG. 28.This highlights the transparent substrate for the colour conversionlayer and the temporary wafer used to support the thin GaN LED layer.

In FIG. 30, the colour converter is attached to the rest of the device.Therefore, once suitable alignment has been completed the two layers arebrought into contact and bonded together. As an example, this may be anepoxy type bonding process. It is also possible to perform this bond ofthe layer to provide a localised hermetic seal between the twosubstrates, thus providing enhanced protection to the colour conversionlayer. In a preferred embodiment the colour converter is pumped withblue light and there is a red/green colour conversion layer which is thephosphor layer. Alternatively, the layer is a quantum dot ortransparent/diffusing layer (blue) or a mixture thereof. FIG. 30 showsthat extending vertically down from the glass substrate there areopaque/black mask or reflective masks 28. The reflective mask 28 ispreferred as this has the ability to recirculate the light, minimisescrosstalk and enhances display contrast. The reflective mask 28therefore has the ability to transmit blue light and reflect red andgreen light when a filter is placed before the layer. In an alternative,if the filter is placed after the layer then the blue light will berecirculate and the red and green transmitted.

In FIG. 31, the temporary layer 13 is removed from the combined layers.This may be achieved by heating, solvent and or any other standardtechnique.

In FIG. 32, an electronic drive layer 39 is brought up to and alignedwith the metal bond pads 11. The electronic drive layer 39 is a CMOS,TFT or NMOS layer. In the Figure this is showing the bond stack withtopology for clarity. In the preferred embodiment the GaN modified LEDscan be used with a planarised surface. (FIG. 9 description is the way toprovide a flat smooth surface). With no or little topology a range ofdifferent control backplanes can be bonded to the GaN surface includingbut not limited to CMOS, NMOS, TFT, etc.

In FIG. 33 the metal bond pads 39 a are attached to the bond pads 11.This is the completed micro-display structure highlighting many of thefeatures necessary to provide display capability.

FIG. 34 is a plan view of pixel layout implementation in layout A. Inthe embodiment shown the three sub-pixels 43 are arranged in a 20×20micron cell. This highlights the possible layout of the GaN fedsub-pixels. In this particular configuration, three LEDs 43 are placewithin the, as an example, 20 μm×20 μm pixel region.

In FIG. 35 the bond pads 39 a are shown as being positioned away fromthe pixels 43. The bond pads associated with each LED sub-pixel are ingeneral larger. This can distribute the bond force and reduce thelikelihood of physical damage. Furthermore, the resistance can bereduced by increasing the bond pad dimension. Using techniques such asGaN mod definition of the sub-pixel leads to improved performance as itreduces topology on the GaN surface and can provide LED sub-pixels withwell-defined isolation layers.

FIG. 36 is a plan view of pixel layout implementation in layout B. Inthe embodiment shown there are four sub-pixels 44 included in eachpixel. This is similar to FIG. 34 but in this instance four GaN ledsub-pixels 44 are defined within the 20 μm×20 μm pixel region.

In FIG. 37 the bond pads 46 may be positioned over the pixel 44. This issimilar to FIG. 35 with larger bond pads over each sub-pixel shown inFIG. 36.

In FIG. 38 there is a red colour conversion cell 47, a green colourconversion layer 48 and a blue pixel without colour conversion cell 49(which may include a transparent/diffusing layer). This represents thelayout of colour conversion layer in relation to sub-pixel layout inFIG. 34.

In FIG. 39 there is shown an opaque/reflecting matrix 50. This issimilar to FIG. 38 also showing the matrix isolating each sub-pixel.

In FIG. 40 there is a matrix of red colour conversion cells 47, greencolour conversion cells 48 and blue pixels without colour conversioncells 49. This shows the layout of the colour conversion layer inrelation to sub-pixel layout in FIG. 36.

In FIG. 41 there is a matrix of red colour conversion cells 47, greencolour conversion cells 48, blue pixels without colour conversion cells49 and an opaque/reflecting matrix 50. This is similar to FIG. 40 andshows the matrix isolating each sub-pixel.

Whilst specific embodiments of the present invention have been describedabove, it will be appreciated that departures from the describedembodiments may still fall within the scope of the present invention.For example, any suitable type of colour converter may be used and anysuitable type of LED.

1-68. (canceled)
 69. An integrated light emitting diode (LED)micro-display, comprising: a color converter configured to changewavelength of light, the color converter including: a color conversionlayer; and a long pass filter on the color conversion layer, wherein thelong pass filter allows converted light by the color conversion layer topass from the color conversion layer, and recycles unconverted light; amicro-LED array connected to the color converter, the LED arrayconfigured to form electrical connections and pump light into the colorconverter; and a backplane controller for the micro-LED array includingan electronic drive layer, the color conversion layer of the colorconverter producing light at a longer wavelength than the pumped lightfrom the micro-LED array.
 70. The integrated LED micro-display of claim69, wherein the color conversion layer is made from at least one ofphosphor, quantum dot, or organic substance.
 71. The integrated LEDmicro-display of claim 69, further comprising a substrate, and whereinthe color conversion layer is on the substrate.
 72. The integrated LEDmicro-display of claim 71, wherein the substrate is made from glass,sapphire, silicon, gallium nitride (GaN), or silicon carbide.
 73. Theintegrated LED micro-display of claim 71, further comprising atransparent layer on the substrate.
 74. The integrated LED micro-displayof claim 73, further comprising a mask formed between the colorconversion layer and the transparent layer, wherein the mask is opaqueor reflective.
 75. The integrated LED micro-display of claim 69, whereinthe color converter further includes a short pass filter on a side ofthe color conversion layer opposite the long pass filter, the short passfilter transmits the pumped light from the micro-LED array to the colorconversion layer and reflects the light at the longer wavelength. 76.The integrated LED micro-display of claim 69, where in the colorconverter further includes: a substrate having a first and second side;opaque/reflective features on the first side of the substrate; amodified refractive index transparent layer which provides an opticalwaveguiding function on the second side of the substrate; and anun-modified refractive index layer on the second side of the substrate.77. An integrated light emitting diode (LED) micro-display comprising: acolor converter configured to change wavelength of light, the colorconverter including: a modified refractive index layer; an un-modifiedrefractive index layer; and a mask that is opaque or reflectivepositioned between the modified refractive index layer and un-modifiedrefractive index layer; and a micro-LED array connected to the colorconverter, the micro-LED array configured to form electrical connectionsand pump the light into the color converter; and a backplane controllerfor the micro-LED array including an electronic drive layer, the colorconverter producing light at a longer wavelength than the pumped lightfrom the micro-LED array.
 78. The integrated LED micro-display of claim77, wherein: the color converter further includes: a color conversionlayer configured to produce the light at the longer wavelength than thelight from the micro-LED array; and a transparent layer configured totransmit the light from the micro-LED array; and the modified refractiveindex layer and the un-modified refractive index layer are positioned toform a waveguide for the light produced by the color conversion layerand transmitted by the transparent layer.
 79. The integrated LEDmicro-display of claim 78, wherein the mask is between the colorconversion layer and the transparent layer.
 80. The integrated LEDmicro-display of claim 78, wherein the color conversion layer is madefrom at least one of phosphor, quantum dot, or organic substance. 81.The integrated LED micro-display of claim 78, wherein the colorconverter further includes: a substrate including a first side and asecond side, the modified refractive index transparent layer and theun-modified refractive index layer on the second side of the substrate;and opaque/reflective features on the first side of the substrate. 82.The integrated LED micro-display of claim 81, wherein the substrate ismade from glass, sapphire, silicon, gallium nitride (GaN), or siliconcarbide.
 83. The integrated LED micro-display of claim 78, wherein thecolor converter further includes a short pass filter on the colorconversion layer, the short pass filter transmits the light from themicro-LED array to the color conversion layer and reflects the light atthe longer wavelength.
 84. The integrated LED micro-display of claim 78,wherein the color converter further includes a long pass filter on thecolor conversion layer, the long pass filter allows converted light bythe color conversion layer to pass from the color conversion layer, andrecycles unconverted light.
 85. An integrated light emitting diode (LED)micro-display comprising: a micro-LED array configured to generatelight; and a color converter coupled to the micro-LED array, the colorconverter configured to change wavelength of the light, the colorconverter including: a color conversion layer positioned to receivefirst light from the micro-LED array and generate second light having alonger wavelength than the first light; a transparent layer positionedto receive third light from the micro-LED array and transmit the thirdlight; a mask that is opaque or reflective positioned between the colorconversion layer and the transparent layer; and a modified refractiveindex layer and an un-modified refractive index layer positioned to forma waveguide for the second light from the color conversion layer and thethird light from the transparent layer.
 86. The integrated LEDmicro-display of claim 85, wherein the color converter further includes:a substrate including a first side and a second side, the modifiedrefractive index transparent layer and the un-modified refractive indexlayer on the second side of the substrate; and opaque features on thefirst side of the substrate.
 87. The integrated LED micro-display ofclaim 86, wherein the waveguide reduces divergence of light transmittedthrough the modified refractive index layer.
 88. The integrated LEDmicro-display of claim 86, wherein the color converter further includesa short pass filter on the color conversion layer, the short pass filtertransmits the first light from the micro-LED array to the colorconversion layer and reflects the second light at the longer wavelength.